As a damping force control apparatus of a suspension, for example, a technique described in JP10-913A (Reference 1) has been known.
The damping force control apparatus according to the technique includes an opening determination unit that determines an orifice opening degree of a damper for controlling a damping force. The opening determination unit determines the orifice opening degree based on a vehicle velocity and a relative velocity of a vehicle wheel with respect to a vehicle body. Here, each of the vehicle velocity and the relative velocity is calculated based on an output (acceleration) that is obtained by an acceleration sensor of the vehicle body.
The relative velocity is estimated by applying a Kalman filter theory to a modeled damper. Upon application of the Kalman filter theory, the damping force of the damper is divided into a linear component and a nonlinear component.
In addition, in JP2010-58541A (Reference 2), an output deviation, corresponding to the difference between a sprung velocity (observed output) of the vehicle and an estimated sprung velocity (estimated observed output) from an approximate vehicle model of an observer for the actual vehicle model state amount estimation, is obtained. Then, a first observer gain of an observer for the actual damping force estimation calculates a dynamic characteristic compensation signal and a second observer gain of the observer for the actual vehicle model state amount estimation calculates a vehicle model compensation signal from the output deviation. The dynamic characteristic compensation signal is input into a dynamic characteristic giving unit of the observer for the actual vehicle model state amount estimation and is used for adjusting setting contents of the dynamic characteristic giving unit. Thus, the occurrence of time delay of the control is suppressed.
However, in the estimation of the relative velocity of the above-described related art, the estimated relative velocity is greatly diverged from the actual measured relative velocity. For example, if the observer of the relative velocity is designed to improve estimation accuracy in a case of full soft (state where a damping coefficient is the smallest), estimation accuracy in a case of full hard (state where the damping coefficient is the largest) is reduced. In contrast, if the observer of the relative velocity is designed to improve the estimation accuracy in a case of full hard, the estimation accuracy in full soft is reduced. As a result, the damping force of the damper may not be set to be an appropriate value on a full hard side (or on a full soft side). As described above, there is room for improvement in the damping force control apparatus of the suspension of the related art.